The known turning tool shown in FIGS. 1-5, includes a basic body 1, in the form of a so-called shaft holder, a turning insert 2 for chasing threads, a shim plate 3 for the turning insert, as well as two tightening elements 4, 5 for individual clamping of the turning insert and the shim plate. In order to house the turning insert and the shim plate, a pocket, in its entirety designated 6, is formed in the basic body 1 and opens in a front surface 7 as well as in a side surface 8 of a front head 9, which at the rear transforms into a thinner shaft 10 having a polygonal cross-section shape.
The pocket 6 accommodates two spaces spaced-apart level-wise, a lower one of which has the purpose of housing the shim plate 3, and is delimited by a plane bottom surface 11 as well as by two side support surfaces 12, 13 (see FIGS. 2 and 3), which suitably are plane. Among the side support surfaces, the surface 12 is longest and forms a primary side support surface, while the other one is comparatively short and forms a secondary side support surface 13. As is seen from the end borderlines 14 in FIG. 2, the surfaces 12, 13 are inclined in the upward/inward direction in relation to the bottom surface 11, the angle between the individual side support surface and the bottom surface being acute, e.g. within the range of 87-89.5°. The side support surfaces 12, 13 are delimited upward by straight borderlines 15, which run parallel to the bottom surface 11.
The upper space for the turning insert 2 is delimited by two upper side support surfaces 16, 17 against which the turning insert is urgable, and which are inclined in the direction inward/upward like the lower side support surfaces (see the end borderline 18 in FIG. 2). Each upper side support surface is delimited upward by a borderline 19, which runs parallel to the bottom surface 11. All side support surfaces 12, 13, 16, 17 converge into a widened clearance space, designated 20, in the interior of the pocket. An outer corner section 11a of the bottom surface 11 is delimited by two borderlines 21, 22, which extend at approximately a right angle to each other and form transitions to the front and side surfaces 7, 8 of the basic body.
The turning insert 2 has a triangular basic shape and comprises an upperside 23 and an underside 24, between which there are extending three side surfaces 25 that in this case are homologous and situated in an imaginary, equilateral triangle and therefore converge in pairs at an angle of 60° toward three corners. In this case, the underside 24 of the turning insert is formed with a central, hexagonal part surface 24a, which is countersunk in relation to three plane, foot-like part surfaces 24b at the corners of the turning insert. In each corner, a tooth 26 is formed having a cutting edge 27, which in the embodiment shown is acute and includes two V-shapedly converging part or flank edges 27a, 27b. Furthermore, in this case the turning insert is formed with a through hole 28 for the tightening screw 4.
The shim plate 3 includes an upperside 29 and an underside 30 between which a number of side surfaces extend, viz. a pair of primary and secondary contact surfaces 31, 32 (see also FIG. 4), which are active so far that they can be urged against the primary and secondary side support surfaces 12, 13 in the pocket 6, as well as a pair of inactive surfaces in the form of an outer end surface 33 and an outer side limiting surface 34, which together delimit a corner section located above the free, outer corner section 11 a of the bottom surface 11. In this case, the end surface 33 transforms into a countersunk shoulder surface 35 against which the head of the tightening screw 5 is urgable. The shim plate 3 also includes a through hole 36 for the screw 4. The two contact surfaces 31, 32 converge into a marked tip 37.
In the bottom surface 11 (see FIG. 3), a hole 38 mouths in which a female thread is formed for the co-operation with the male thread of the screw 4. In the side surface 8 of the basic body 1, a seat having a bottom surface 39 is countersunk, in which bottom surface a hole 40 having a female thread mouths. The bottom surface 39 forms a stop for the head of the screw so far that it makes further tightening of the screw impossible if the head would reach up to the same.
A characteristic feature for the known tool is that the two side contact surfaces 31, 32 of the shim plate 3 converge toward the common tip 37 at the same convergence angle as the side surfaces 25 of the turning insert, i.e., 60°. For this reason, the individual lower side support surface 12, 13 (see FIG. 3) is parallel to the respective side support surface 16, 17 of the turning insert. In other words, the shim plate 3 is wedged up between the lower side support surfaces 12, 13 in the same way as the turning insert is wedged up between the upper side support surfaces 16, 17. The clamping of the shim plate is provided by the screw 5, while the clamping of the turning insert is effected by the screw 4. In order to fulfill this task, in this case the screw 4 is tiltable (via cone surfaces, not shown, down in the hole 38), so that the same, upon final tightening, is tilted inward toward the clearance space 20, the conical head of the screw, which is smaller than the hole 28, being urged against the cone surface 41 in the hole 28 (see FIG. 2) to apply an inwardly directed force to the turning insert. The tightening screw 4 in question also has the advantage that the same allows removal of the turning insert without being unscrewed out of the hole 38. By the tiltability thereof, the screw can be tilted outward after having been unscrewed some thread turns without loosing the engagement with the female thread, whereby the head of the screw can freely pass through the hole 28 of the turning insert.
In FIG. 1, the arrow P indicates the feeding direction of the tool during chasing. When the active cutting tooth 26 enters into the workpiece at the entrance of the thread to be made, the workpiece is first entered by the part edge 27a, a force being applied to the turning insert, which force aims to turn the same clockwise around the tightening screw 4. As soon as also the second part edge 27b also has entered the workpiece so that a full thread is turned, the cutting forces on the two part edges will become essentially equally large. When the cutting tooth finally leaves the workpiece, the part edge 27a is detached before the part edge 27b. This means that the interaction of forces is inverted so far that a force is applied to the turning insert, via the part edge 27b, which aims to turn the insert counter-clockwise around the screw 4.
A disadvantage of the known tool construction is that the fixation of the turning insert may become unstable and unreliable, among other things as a consequence of the fact that the only active structure that, besides the tightening screw, counteracts turning of the turning insert is the two upper, converging side support surfaces in the pocket. Thus, the underside of the turning insert and the upperside of the shim plate are plane and smooth, and therefore they lack the capability of locking the turning insert. Considering the rule that small convergence angles gives a good wedging effect, and vice versa, the convergence angle of 60° of the side surfaces of the turning insert is furthermore comparatively large. Even if the stability problems are moderate at the entering of the turning insert into the workpiece, as the cutting forces aim to turn the turning insert in such a way that the forces essentially are carried by the primary, long side support surface, the problems become marked when the turning insert leaves the workpiece and is subjected to forces that primarily are to be carried by the short, secondary side support surface. In the latter case, the risk is large that the short contact surface of the turning insert slides along the co-operating, secondary side support surface during tensioning of the tightening screw and dislodging of the turning insert.
If the turning insert on and off is dislodged from the desired, predetermined position thereof, the result becomes an impaired dimensional accuracy of the thread turned. Moreover, in polar cases, breakdowns may occur.
In this connection, the use of shim plates in thread-chasing tools is conditioned by the need to vary the tilt angle of the cutting tooth in relation to the workpiece, e.g., when threads having different pitches and diameters, respectively, are to be turned. Therefore, instead of holding in stock a great number of turning inserts having different tooth angles, in practice use is made of a universal type of turning inserts that is combined with different shim plates, which by having different angles between the uppersides and undersides thereof, offer the possibility of altering the tilt angle of the tooth in relation to the basic body (in practice, the tilt-angle variations are about a few degrees).
Related art thread-chasing tools having triangular turning inserts are disclosed in, for instance, European patent documents EP 0119175 and EP 0178273. However, in these cases, the tools lack shim plates.
As disclosed in U.S. Pat. No. 5,810,518, there is a turning tool for, among other things, thread turning that makes use of a triangular turning insert, which is mounted on the basic body via a shim plate. However, in this case, the shim plate is locked in relation to the basic body as well as the turning insert via serration surfaces, which individually include a plurality of ridges and grooves, and furthermore, support for the side surfaces of the turning insert are lacking. For these reasons, the manufacture of the tool is complicated at the same time as the risk of form defects in the turning insert as well as the shim plate may give rise to poor machining precision of the tool.
The present invention aims at obviating the above-mentioned shortcomings of previously known turning tools and at providing an improved turning tool of the type that makes use of shim plates to determine the tilt angle of the cutting tooth. Therefore, an object of the invention to provide a tool, the turning insert of which is kept stably fixed in the desired, predetermined position thereof also under varying stresses on the insert.
Another object of the invention is to provide the improved stability using simple and few ways, which guarantees a continued simple and cost-effective manufacture of the turning insert as well as of the shim plate. Among other things, the requirements of precision when forming the pocket in the basic body should be moderate.
Yet another object of the invention is to increase the freedom of the designer to form space-requiring teeth having a complicated shape at the corners of the turning insert, more precisely by shortening of the side surfaces of the turning insert.